1. Field of Invention
The present invention is related to a traditional DC-AC single-phase full-bridge inverter. Particularly, the present invention is related to a method of operating a switching device in an inverter in a soft switching mode so as to promote efficiency of an inverter, reduce a rating value required for the switching device, reduce electromagnetic interferences caused by switching operations on the switching device and increase switching frequency of the switching device.
2. Related Art
With rapid development of semiconductor technology used in integrated circuits, electronic products are marching to have the features of slightness and compactness. For a current high efficiency inverter, it typically has an operating frequency of over than 20 kHz so as to stay away from an audio frequency band. When a hard switching manner is used, a power transistor in the inverter will have a greater switching loss as switching frequency thereof increases, resulting in a greater power loss. In this case, a considerable large heat sink is required for heat removing. As such, not only volume of the heat sink has to be increased but also efficiency of the inverter is reduced. To resolve this problem, a soft switching method has been suggested to be utilized. FIG. 1 is a schematic diagram of a prior power switch upon which the hard switching method is applied. In this case, a voltage VDS across on the transistor will rise abruptly and a voltage pulse will thus take place when the switch is cut off. This voltage pulse may have a value greater than an input voltage VSLP of the power switch and increase the voltage stress of the power switch. On the other side, when the switch is turned on, a current pulse greater than a normal current ISLP is arisen at an instant after the switch is turned on due to an output capacitor of the switching device, resulting in an increase stress of the power switch. In summary, the power switch either has a non-zero voltage or has a non-zero current irrespective of the turn-on or cut-off state itself and thus a pulse bringing about a power loss is generated in any state. Accordingly, switching loss may be seen at a time when current alternation occurred in the power switch no matter turn-on or cut-off state is stayed, shown in FIG. 1.
Referring to FIG. 2, waveforms of a voltage and a current of the-prior power switch when a hard switching operation is conducted thereon are shown therein. As shown, upon the power switch being turned on, the current thereof increases. In a tr period where the current begins and increases, a current spike is arisen owing to an output capacitor and stray capacitance of the power switch. However, since the voltage at this moment is still very high, which is equal to an input voltage VSLP. Therefore, a power loss is caused during the time period tr. On the other hand, when the power switch is cut off, a voltage drop between the power switch thereof increases abruptly and a voltage spike is generated. However, since the current IDS of the power switch has not decreased down to zero at this time, a lower loss is also resulted during the time period tf when the power switch is cut off.
Referring to FIG. 3, a schematic diagram showing waveforms of a voltage and a current of a power switch when a soft switching operation is conducted thereon is illustrated therein. As shown, after the power switch is turned on and when the current begins and increases during a time period of tr, the voltage of the power switch is zero. This behavior is generally called zero voltage switching (ZVS). On the other hand, when the power switch is cut off, the voltage rises only after the current is decreased to zero. This behavior is called zero current switching (ZCS). In this manner, switching loss is greatly reduced and overall efficiency of the inverter is thus promoted. In view of the above, a main difference between the hard and soft switching methods is the state when the power switch is switched. The differences are listed in Table 1.
The soft switching technology suitable to be used in a DC-AC inverter may be achieved by the following three configurations, which are classified based on their architectures: (1) Addition of a resonance network at the load side of the inverter, (2) Addition of a resonance network provided at a bridge of the inverter and (3) Addition of a resonance network at a DC link of the inverter. These three configurations of inverter will be explained as follows.
<The Soft Switching Method Applied in a Configuration where a Resonance Network is Added at the Load Side of the Inverter>
This kind of soft switching method may be further classified into two methods based on if a series resonance network or a parallel resonance network is added. In the series resonance network configuration, a square-wave voltage is transmitted from a bridge of the inverter to the series resonance network. Examples of such series resonance network configuration are provided herein for reference. F. C. Schwarz set forth “A method of resonant current pulse modulation for power converters,” IEEE Trans. on Industrial Electronics, Control and Instrument, Vol. IECI-17, pp. 209–221, June 1970. Kifune, H.; Hatanaka, Y; Nakaoka, M. disclosed “Quasi-series-resonant-type soft-switching phase shift modulated inverter”, IEE Proceedings—Electric. Power Applications, Volume: 150, Issue: 6, 7 Nov. 2003, Pages: 725–732. The two prior arts are related to a configuration where the load and the series resonance network are connected in series. N. Mapham proposed “An SCR converter with good regulation and sine-wave output,” IEEE Transactions on. Industrial Generation Application, Vol. IGA-3, pp. 176–187, March/April 1967. Chien-Ming Wang set forth “Nonlinear-controlled strategy for soft-switched series-resonant DC/AC inverter without auxiliary switches”, IEEE Transactions on Power Electronics, Volume: 18, Issue: 3, May 2003, Pages: 764–774. The two prior arts are related to a configuration where the load and the series resonance network are connected in parallel. Since the series resonance manner is used to obtain the result of zero current switching, a main limitation of the corresponding configuration lies in that the switching frequency may not be higher than the resonance frequency. Further, regulation of the output voltage or current of the inverter may become worse when resonance frequency of the power switch shifts owing to aging or production inconsistency of the power device occurred.
In the parallel resonance network configuration, a square-wave current is transmitted from a bridge of the inverter to the parallel resonance network. J. G Kassakian set forth “A new current mode sine wave inverter,” IEEE Transactions on Industrial Applications, Vol. 18, pp. 273–278, May/June 1982. V Chudnovsky, B. Axelrod and A. L. Shenkman disclosed “An approximate analysis of a starting process of a current source parallel inverter with a high-Q induction heating load,” IEEE Transactions on Power Electronics, Vol. 12, pp. 294–301, March 1997. The two prior arts are related to a configuration where the load and the parallel resonance network are connected in parallel. M. K. Kazimierczuk and R. C. Cravens II proposed “Current-source parallel resonant DC/AC inverter with transformer,” IEEE Transactions on Power Electronics, Vol. 11, pp. 275–284, March 1996, which are also related to the configuration where the load and the parallel resonance network are connected in parallel. Since an AC voltage may be caused in the power switch owing to the parallel resonance network, the power switch has to be provided with capability of isolating a reverse voltage or has to be added additionally with a diode to block this reverse voltage.
In short, the soft switching purpose may be achieved by addition of a resonance network at the load side. The configuration with the resonance network added at the load side is more suitable to be used when a fixed load is utilized. In the case of a large load variation occurred, this kind of soft switching techniques will perform poorer.
<Soft Switching Method Applied in a Configuration where a Resonance Network is Added at a Bridge of the Inverter>
For the configuration where a resonance network is added at a bridge of the inverter, an input voltage of the inverter has a fixed voltage or current. R. Tymerski, V. Vorp'erian, and F. C. Lee set forth “DC-to-AC inversion using quasiresonant techniques,” Transactions on Power Electronics, Vol. 4, pp. 381–390, October 1989, which is a configuration where quasi-resonant zero voltage switching technology is used. To achieve the zero voltage switching, the power switch has to endure a greater current. Further, performance of this method is dependent on the resonant inductance and it is difficult to achieve the purpose of zero voltage switching for some inductance values. In addition, this method is not suitable to be used where an inductance load is used at an output of such as a motor-driven control. J.-S. Lai, R. W Young, G. W. Ott, Jr., J. W. McKeever, and F. Z. Peng, disclosed “A delta-configured auxiliary resonant snubber inverter,” IEEE Transactions on. Industrial Applications, Vol. 32, pp. 518–525, May/June 1996. Smith, K. M., Jr.; Smedley, K. M. proposed “Lossless passive soft-switching methods for inverters and amplifiers”, IEEE Transactions on Power Electronics, Volume: 15, Issue: 1, January 2000, Pages: 164–173. In the two prior arts, the soft switching purpose is achieved by providing additionally a resonant snubber. An advantage of such circuit is that the circuit may be operated in cooperation with a pulse width modulation method. However, a passive resonant snubber is necessary and complexity of the circuit is increased. Smith, K. M., Jr.; Smedley, K. M. set forth “Intelligent magnetic-amplifier-controlled soft-switching method for amplifiers and inverters”, IEEE Transactions on Power Electronics, Volume: 13, Issue: 1, January 1998, Pages: 84–92, in which a resonant circuit is added at a bridge of the inverter to achieve the purpose of soft switching. However, an auxiliary switch and a resonant circuit comprising passive devices for resonance are required to be additionally provided, increasing relatively the cost of the inverter.
In short, such inverter with the resonance circuit added at one of the bridges thereof is achieved with respect to the purpose of soft switching mostly based on the provision of the auxiliary switch. Such kind of inverter is operated based on the principle where the input and the load thereof, when zero voltage switching is to be achieved, have to form an parallel resonance circuit when the auxiliary switch is turned on, while the input and the load thereof, when zero current switching is to be achieved, have to form an series resonance circuit when the auxiliary switch is turned on. In addition, control timing for the auxiliary switch of such configuration has to be particularly designed, increasing complexity of the associated circuit.
<Soft Switching Method Applied in a Configuration where a Resonance Network is Added at the DC Link of the Inverter>
The soft switching method applied in a configuration where a resonance network is added at the DC link of the inverter may be classified into two types, including one having an AC voltage and current at a DC link side and one having a DC voltage and current at the same. P. K. Sood and T. A. Lipo, set forth “Power conversion distribution system using a high-frequency AC link,” IEEE Transactions on. Industrial Applications, Vol. 24, pp. 288–299, March/April 1988, in which a DC link resonance network is used to resonate the input voltage into an AC form so as to the achieve the purpose of soft switching. In such configuration, a cyclocoverter mode is utilized. A disadvantage of such configuration is that the switch has to endure the AC voltage and the output may not be applied with the commonly used pulse width modulation method but requires to be processed by means of discrete pulse modulation (DPM). Yie-Tone Chen disclosed “A new quasi-parallel resonant DC link for soft-switching PWM inverters”, IEEE Transactions on Power Electronics, Volume: 13, Issue: 3, May 1998, Pages: 427–435. Xiangning He; Kuang Sheng; Williams, B. W.; Zhaoming Qian; Finney, S. J. proposed “A composite soft-switching inverter configuration with unipolar pulsewidth modulation control”, IEEE Transactions on Industrial Electronics, Volume: 48, Issue: 1, February 2001, Pages: 118–126. The two prior arts achieve the purpose of soft switching by adding a parallel quasi-resonant network at the DC link side. According to this method, a negative voltage will never present at the DC link side and hence the switch does not necessarily have to endure an AC voltage. In achieving this purpose, the auxiliary switch has to be additionally provided, which requires more complex control timings.
In view of the above, shortcomings are encountered in the prior arts. In this regard, the inventors of this invention have engaged in intensive research and tried to look for efficient and practicable solutions. After many tests and efforts, a modified high-efficiency phase-shift modulation method is set forth.